It was later owned by his business partner William Pirrie, who became chairman of Harland & Wolff. It sits on a site of approximately 13 acres.

Stormont's Department of Finance and Personnel bought it on behalf of the Assembly Commission from the Police Authority - the predecessor of the Policing Board - in September 2001 for £9.1m.

Last year Ulster Unionist leader Mike Nesbitt suggested Ormiston House could be a venue for a new international trauma centre.

But despite owning it for 13 years, the Assembly never made use of it. It lay empty from 2001 until the Assembly sold it in November 2014 for just £1.5m, suffering a huge loss. The Assembly spent around £1.5m on security and maintenance.

Earlier this year DUP MLA Jim Wells - who was a member of the Assembly Commission in 2001 - claimed that an obstacle to the property being used was that people living nearby had concerns about it being used as Government offices.

"So there were delays in the planning and then it wasn't needed," he said.

"We had no idea that there would be difficulty in the planning and that there would be such opposition from residents."

In response to a Freedom of Information request by the Belfast Telegraph, the Assembly said it has not investigated the matter, but simply said it will learn from the experience.

"The Assembly Commission has no plans to acquire additional property but will draw on this experience should such arise in the future," a spokeswoman said.

Now the Belfast Telegraph can reveal that Ormiston House is being converted back into a residence. The owner has applied to Belfast City Council for planning permission for a dwelling, rather than for business use.

A City Council report described the building as listed, two storeys in height and located within extensive grounds comprising grassed lawns and a large number of mature trees.

The report noted that in 2012 planning permission was granted to alter the house, gatehouse and mews buildings to provide 11 apartments, plus a new block consisting of 20 apartments. This was never followed through.

Ormiston House was built in 1867 as a Scottish baronial-style mansion house with crow-stepped gables, a bartizan turret and gargoyles.

It passed into the ownership of Harland & Wolff following the death of Lord Pirrie in 1924 and was sold to Campbell College school in 1928, which held it until the mid-1970s.